Improve Executor tool guidance

[khmichaelkim]

Summary

• direct models to the companion skills MCP tool without suggesting a nonexistent sandbox function
• add compact inputConstraints to tools.describe.tool() for numeric, collection, and string validation limits
• clarify outer Executor failures versus nested MCP application status
• cover JSON Schema/OpenAPI exclusives, references, tuples, cycles, and composition semantics

Testing

• bun run format:check
• bun run lint
• bun run typecheck
• bun run test
• bun run --cwd e2e test:selfhost -- scenarios/tool-descriptions.test.ts